Digital adaptive reduction of noise in the time domain is typically performed by sampling the analog output of a microphone that is appropriately positioned to sense the input noise. The sampled analog noise is then converted to digital format via an A/D converter, passed through an adaptive digital filter and then converted back to analog via a D/A converter before being output to a speaker. The analog output of a microphone is utilized as the input to the internal adaptive algorithm within the prior art noise reduction system.
Since the group delay of such a system as described above is relatively large, a typical method of noise reduction employed in the prior art using such a system is to reduce the effect of periodic noise sources as opposed to arbitrary noise sources.
Typical prior art noise reduction systems utilize an adaptive digital filter in the main data path to reduce the effect of the noise source. This causes the group delay of the system to be relatively large.
For example, U.S. Pat. No. 5,553,154, issued to Tamamura et al., discloses an adaptive filter that receives pulses that are synchronized with the period of the noise. The interval of the input noise determines the length of the taps in the delay line of the adaptive filter.
U.S. Pat. No. 5,613,009, issued to Miyazaki et al., discloses a vibration control system that a reference signal from the vibration source and an error signal from an object are input to the adaptive filter. Feedback means generates a feedback control signal in combination with a feedfoward signal, forms a drive signal for a vibrating means.
U.S. Pat. No. 5,627,896, issued to Southward et al., discloses an active system for controlling noise. The system operates by limiting the output gain from a gradient descent algorithm.
Frequency domain analysis of the input data in the main data path of the system is another common technique utilized by prior art noise reduction systems. For example, U.S. Pat. No. 5,271,062, issued to Sugita et al., discloses a noise reduction system that generates a signal having the same frequency but inverted phase relative to the input noise.
U.S. Pat. No. 5,347,586, issued to Hill et al., discloses a noise control system comprising a reference microphone for generating a reference signal that is correlated with the noise emanating a noise source, a plurality of loudspeakers, a plurality of error microphones for generating a plurality of error signals and an adaptive controller.
U.S. Pat. No. 5,365,594, issued to Ross et al., discloses a vibration control system that utilizes a vibration input signal derived from a sensor sampling the vibrations to generate an output representative of the interference free vibration of a primary source.
U.S. Pat. No. 5,519,637, issued to Mthur et al., discloses an active structural acoustic control method for reducing the sound emitted through a structure. The method uses an array of transducers placed in a far field structure and an array of actuators mounted or embedded in that structure. Each is controlled by the system controller and uses reference signals derived from the noise source.
Alternatively, another method of noise cancellation used in prior art systems places the microphone as close to the noise source as possible and the loudspeaker relatively far from the microphone so as to create a delay equal to the time for the noise to travel from the microphone to the speaker. This delay is intentionally created in order to match the internal signal processing time of the noise reduction system. The propagation time for the noise is configured to roughly match and compensate for the signal propagation time within the noise reduction system. This noise reduction method is particularly useful for cancellation of noise in a duct such as an air conditioning duct. The internal signal processing is performed during the time that it takes for the sound waves to travel from the microphone to the loudspeaker.
Another prior art noise reduction technique related to the one just described, is to place the speaker close the noise source rather than far away from it, place a second microphone in the desired quiet zone and to adapt a digital filter utilizing the second microphone output. However, this method is useful for canceling repetitive noise only.
The implementation and calibration of noise reduction systems made according to the prior art techniques described above are typically very difficult and correspondingly costly due to the need to supply a plurality of different elements, e.g., two microphones and a loudspeaker, and place these plurality of elements in their appropriate places.
Another disadvantage of prior art noise reduction systems is that if the canceled noise is not periodic in nature, then the physical dimensions of the cancellation system become relatively large. When the loudspeaker is located far away from the noise source, then the noise cancellation is localized and limited to a relatively small area. It would thus be desirable to have a noise reduction system in which the physical design and calibration limitations of prior art noise reduction systems are avoided.
U.S. Pat. No. 5,410,607, issued to Mason et al., discloses a noise cancellation system for reducing noise radiated from a complex vibrating surface. The system includes a motion sensor, a controller having fixed transfer function and operative to generate an antinoise signal, and an acoustic driver operative to generate an acoustic antinoise field that is substantially 180 degrees out of phase with the original noise field. The antinoise field reduces the original noise field using destructive interference.
U.S. Pat. No. 5,618,010, issued to Pla et al., discloses an active noise reduction system that utilizes a noise radiating panel to generate a noise that cancels the noise source. A piezoceramic actuator is connected to the panel. A sensing device such as an accelerometer is used in generating an opposite noise signal with which to cancel the noise source in the vicinity of the panel.